FIG. 18 illustrates an aircraft fuel vent pipe 40. If the aircraft fuel tank is overfilled with fuel then the vent pipe 40 is arranged to enable excess fuel to flow out of the fuel tank. For this reason the vent pipe 40 extends downwardly and lies flush with the aerodynamic outer surface of the lower skin of the aircraft wing. Thus the open end of the pipe is exposed to an aerodynamic cross flow of air 41 during flight of the aircraft.
The cross flow generates an instability 42 which generates a strong acoustic tone which emanates from the pipe. It is excited by the natural frequency of the external flow as a function of the local true air speed and the diameter of the pipe. The resonance that occurs is a function of the depth/diameter ratio of the pipe and represents a standing pressure half wave 43.